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Daniel Hernandez <br />April 21, 2006 <br />Page 3 <br />for the dam and landslide after implementation of the preventive measures (also noted in <br />Table A-1 in report). The referenced GEI report states that the yield accelerations for the <br />dam and landslide with the stability berm and buttress fill are 0.13 g and 0.11 g, respectively. <br />Furthermore, it should be noted that the GEI report indicates the relationship between ML and <br />pga is dependent on the horizontal and vertical distance from the seismic event to the <br />location of interest. The maximum potential ML for the dam site is 2.3 with a corresponding <br />pga of 0.06. The anticipated maximum pga for the dam site is 0.08 with a corresponding ML <br />of 2.0. Therefore, establishing M~ alone as a monitoring threshold would be incorrect. We <br />concur with MCC that establishing a value of pga is a more accurate monitoring threshold <br />since this variable is used to identify yield (i.e., a factor of safety of 1.0) in the slope stability <br />analysis. <br />The monitoring threshold after the preventive measures aze in place should be based in terms <br />of peak ground acceleration (pga) and established as the minimum yield acceleration found <br />from slope stability analyses of the dam and landslide. Additionally, we would prefer all <br />seismic monitoring be reported in both M~ and pga. Our office should be notified whenever <br />the pga threshold is exceeded during the monitoring period, whether mine-induced or <br />naturally occurring. <br />Section 2.05.6(61(fl(iv)(A-D) -Detailed Description of Mitieatin¢ Measures <br />Adequacy Item: <br />MCC provided a report prepared by GEI Consultants, Inc. entitled Geotechnical Evaluation <br />ofMine-Induced Seismicity on Monument Dam. The report provides a detailed evaluation <br />of the effects that mining could have on the static and seismic stability on the dam and <br />landslide. A conceptual design ofpreventative measure is presented to increase the factor of <br />safety against slope failure. <br />DWR Response: <br />In general, the report appeazs to follow state-of--the-practice geotechnical and geological <br />engineering methods to analyze the potential impacts of mining on the dam and landslide. <br />The report relies heavily on published literature to estimate the magnitude ofmine-induced <br />seismicity and attenuation relationships to determine peak ground accelerations. A brief <br />independent survey and review of the cited literature indicates the methods and references <br />selected by GEI aze generally accepted by the seismic engineering community. <br />Results of the slope stability analysis of the dam and landslide with the proposed preventive <br />measures improves the static and psuedo-static factors of safety to a level that meets the <br />current minimum standards set forth in the State of Colorado's Rules and Regulation for <br />Dam Safety and Dam Construction. However, the study was conducted using estimated <br />material properties from previous studies and engineering judgment. Construction of the <br />preventive measures will require site specific investigations to accurately portray the soil <br />material properties for engineering design and will be required per the Rules and <br />Regulations. Uvon obtaining soil parameters from a site specificgeotechnical investi atg ion, <br />yield accelerations and factors of safety of the dam and landslide with the stability berm and <br />buttress should be recalculated for use in the eneineerin~ design and to establish monitoring <br />thresholds. <br />